MEMS devices are macroscale devices including a pad that is movable in response to electrical signaling. The movable pad, such as a membrane or cantilevered metal arm, moves in response to an electrical signal to cause an electrical effect. One example is a membrane variable capacitor. The membrane deforms in response to an electrical signal. The membrane itself is part of a capacitor, and the distance between the membrane and another portion of the capacitor changes the capacitance. Another MEMS device is an RF (radio frequency) ohmic switch. In a typical MEMS ohmic switch, application of an electrical signal causes a cantilevered metal arm to either ground or remove from ground state a signal line by completing or breaking ohmic contact with the signal line. Dielectric layers in MEMS devices are used to prevent the membrane, cantilevered arm, or other moving switch pad from making physical contact with other portions of the MEMS device.
MEMS lifetimes continue to be shorter than would make their use widespread. Successes in the range of 1–3 billion “cold” switching cycles have been reported. High frequency applications are especially suited to MEMS devices, but can exceed reported switching cycles in ordinary usage. Also, there is typically a difference between “hot” and “cold” switching lifetimes. “Hot” switching, i.e., a switching test conducted with signals present, is a different measure of operational conditions that usually shows a shorter lifetime than “cold” switching tests would indicate. This is mentioned only to identify that test results are understood with reference to the test conditions. Both types of tests are valid and generally accepted in the art, but only the same types of tests can be directly compared.
A common cause of failure is a stuck switch pad, recognized by experience to be the sticking of the movable switch pad to a dielectric layer. The exact mechanisms for this sticking are not completely understood. Sticking has been attributed to charging of dielectric layers used to isolate electrical contact between the moving switch pad of a MEMS device and an actuation component of the MEMS device. Another common cause of failure and operational inefficiency is the tendency of the switch pad to deform due to spring force. It can move further away from an actuation pad, first leading to an increased voltage required for operation of the switch and eventually leading to a failure.